tudies of genes encoding rabbit MHC (RLA) antigens have ndicated that only a single major transplantation antigen is xpressed in homozygous animals. The RLA gene complex has been ubjected to a detailed analysis at the molecular level in rder to exam this difference from other MHC systems in which hree or more class I antigens are normally expressed. A cDNA ibrary was constructed from a T cell line, RL-5, derived from n inbred rabbit strain. cDNA clones corresponding to four nique transcripts were identified. The clone (pR9) orresponding to the single expressed antigen was represented 6 times among the 31 clones characterized. Two of the cDNA lones have been fully sequenced and analysis of three others s now in progress. A genomic clone corresponding to the xpressed product has been identified and its sequence etermined. These studies have revealed a striking similarity etween human and rabbit class I genes both in the coding egion sequences and in the intron/exon structure. Comparison f rabbit and human class I genes to those of the mouse, on the ther hand, reveal large differences both in exon/intron length nd in sequences found in the 3' coding and noncoding regions. tudies are underway to prepare antibodies directed against the xpressed rabbit class I antigens and to prepare synthetic ligonucleotide probes in order to determine tissue expression f the various members of the rabbit class I family.